Twelve-year-old Mark Scarano thought it was a plastic toy. The Arlington County Police Department apparently thought it was all a joke. But there it was: A 3 1/2-foot cayman, a close relative of an alligator, sunning itself on a rock in the middle of tiny Four Mile Run Creek near Bon Air Park in Arlington.

"I was just walking along the creek and I just saw it laying there," said Scarano, shortly after he spotted the creature yesterday afternoon. "I thought it was plastic, just a decoration. But when I threw a stick at it, it lunged at me."

Scarano, visiting in Arlington from Holton, Maine, scurried the half mile to the North Jefferson Street home of his cousin, Ricardo Zuniga, 12, who "likes nature," said Scarano. The two boys and a friend raced back to the site, with Zuniga dispatched to call the police.

"They didn't believe me at first," said Zuniga. "I told them how old I was and they hung up."

The youngster then called the Animal Welfare League of Arlington which dispatched two officials to the scene where the small creature was still basking. Animal League authorities snagged the thrashing 'gator around the neck with the noose of a control pole, usually reserved for snarring dogs.

"We don't know what we're going to do with it," league director Martha Armstrong said later from the Animal League headquarters where the alligator is being housed in a steel dog cage. She said she immediately began telephoning zoos and nature centers trying to place the creature, which she says has been tentatively identified as a cayman, a close relative of the alligator.

Armstrong speculated the cayman found in the creek may be a pet that escaped. "This one is going to be difficult to place," said Armstrong. She said caymans, native to Central and South America, are fairly common and are frequently sold in pet shops. It is legal to sell caymans as pets she said, while it is against the law to sell its endangered relative, the alligator.

Armstrong said the Animal League is not allowed to place its newest catch as a pet and yesterday was trying to figure out a diet for the hapless cayman. "The guy at the the zoo," said Armstrong, "suggested mice or dry dog food moistened."